Works are in the public domain if they are not
covered by intellectual property rights at all, if the
intellectual property rights have expired, and/or if the
intellectual property rights are forfeited. Examples
include the English language, the formulae of Newtonian
physics, as well as the works of Shakespeare and the
patents over powered flight. (Wikipedia)

American Memory, Library of Congress: " The
Restriction Statement that accompanies each American
Memory collection provides known information regarding
ownership of materials in the collection. If known, we
include contacts for permission. In some cases the
Restriction Statement will indicate that material in a
particular collection may be used freely; in other cases
the Restriction Statement may only be a starting point
for your inquiry."

Beinecke Rare Book & Manuscript Library Digital Images
Online: " The Library may be able to assist in
identifying the copyright holders. It is not necessary
to seek the Library's permission to publish texts or
images (unless the University is identified as the
copyright holder.) The Beinecke Rare Book and Manuscript
Library should, however, be cited as the source."

World War II Posters, Northwestern University: "
Materials published by the U.S. Government Printing
Office are in the public domain and, as such, not
subject to copyright restriction. However, the Library
requests users to cite the URL and Northwestern
University Library if they wish to reproduce images from
its poster database."

WorldImages,
California State University. Includes over
72,000 images that are available free of charge for
educational use. "The internationally recognized
WorldImages database provides access to the
California State University IMAGE Project. It has just
been selected by the Library of Congress
for inclusion in its historic collection of Internet
materials. It contains approximately 80,000 images, is
global in coverage and includes all areas of visual
imagery. WorldImages is accessible anywhere and its
images may be freely used for non-profit educational
purposes. The images can be located using many search
techniques, and for convenience they are organized into
over
800 portfolios which are then organized into subject
groupings."
Portfolio List.